Multilayered sheet

ABSTRACT

This invention pertains to a layered sheet comprising a flame resistant wet-laid nonwoven carrier having a first and second surface and an inorganic refractory layer adjacent to at least one surface of the carrier wherein the refractory layer has a dry area weight of from 15 to 50 gsm, the bond strength between the refractory layer and the surface of the carrier is at least 0.25 lb/in, wherein the carrier comprises from 40 to 70 weight percent of aramid fibers and from 30 to 60 weight percent of polymeric binder, is hydrophilic, has a smoothness on at least one surface of no less than 250 Sheffield units, a thickness of from 0.025 to 0.175 mm and a density of from 0.25 to 0.60 g/cc. Preferably the carrier is in the form of a paper.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/625,839, filed Apr. 18, 2012 which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention pertains to a multilayered sheet comprising a carrier layer and an inorganic refractory layer and a method of making the multilayered sheet. Preferably, the carrier layer is a paper.

2. Background of the Invention

U.S. Pat. No. 6,322,022 to Fay et al. discloses burnthrough resistant systems for transportation especially aircraft.

U.S. Pat. No. 6,670,291 to Tomkins and Vogel-Martin describes a laminate sheet material for flame barrier applications.

U.S. Pat. No. 5,667,886 to Gough et al describes a composite sheet having a substrate layer, a coating layer and a flexible adhesive layer. The substrate layer is preferably a polyester film. The coating layer contains a mineral, preferably vermiculite.

There remains an ongoing need for methods to provide a thin inorganic refractory layer in a form that may be safely handled and subsequently processed into a multi-layer composite for use as a flame barrier component in a thermal and acoustic blanket for aircraft structures.

SUMMARY OF INVENTION

This invention is directed to a layered sheet comprising a flame resistant high strength fiber wet-laid nonwoven paper carrier having a first and second surface and an inorganic refractory layer adjacent to at least one surface of the paper wherein the refractory layer has a dry areal weight of from 15 to 50 gsm, the bond strength between the refractory layer and the surface of the paper is at least 0.25 lb/in, preferably at least 0.8 lb/in, wherein the carrier

(i) comprises from 40 to 70 weight percent of aramid fibers and from 30 to 60 weight percent of polymeric binder,

(ii) is hydrophilic

(iii) has a wet tensile strength of at least 3 lb/in in a first direction and at least 2 lb/in in a second direction, the second direction being transverse to the first direction,

(iv) has a dry tensile strength of at least 7 lb/in in a first direction and at least 3 lb/in in a second direction, the second direction being transverse to the first direction,

(v) has an air permeability no greater than 600 Gurley Air Resistance (sec/100 cc, 20 oz. cyl.).

(vi) has a smoothness on at least one surface of no less than 250 Sheffield units.

(vii) has a thickness of from 0.025 to 0.175 mm,

(viii) has a density of from 0.25 to 0.60 g/cc, and

(ix) has a basis weight of from 20 to 70 gsm.

This invention also pertains to a method of forming a layered sheet followed by subsequent treatment comprising the steps of

(i) depositing an aqueous slurry of inorganic refractory platelets onto one surface of a carrier to form a layered sheet wherein the refractory platelets

-   -   comprise from 7 to 13 weight percent of the slurry,     -   have a particle thickness of from 5 A to 5000 A,     -   have an average diameter of from 15 to 25 micrometers,         wherein the carrier has     -   (a) a wet tensile strength of at least 3 lb/in in a first         direction and at least 2 lb/in in a second direction, the second         direction being transverse to the first direction,     -   (b) a dry tensile strength of at least 7 lb/in in the first         direction and at least 3 lb/in in the second direction,     -   (c) a surface smoothness on at least one surface of no less than         250 Sheffield Units,     -   (d) a thickness of from 0.025 to 0.175 mm,     -   (e) a density of from 0.25 to 0.60 g/cc, and.     -   (f) has an air permeability no greater than 600 Gurley Air         Resistance (sec/100 cc, 20 oz. cyl),     -   (g) has a basis weight of from 20 to 70 gsm, and wherein the         carrier comprises from 40 to 70 weight percent of aramid fibers         and from 30 to 60 weight percent of polymeric binder, and

(ii) drying the layered sheet at a temperature of from 80 to 110 degrees C. until the residual moisture content in the refractory layer is no greater than 10 percent by weight and the bond strength between the refractory layer and the surface of the carrier is no less than 0.25 lb/in.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross section through a multilayered sheet of this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a section through a multilayered sheet 10 comprising a carrier or substrate layer 11 and an inorganic refractory layer 12 deposited onto the carrier layer. A preferred carrier material is a flame resistant high strength fiber wet-laid nonwoven carrier. A preferred carrier is a paper. As used herein, the terms “carrier” and “paper” are used interchangeably.

Carrier

A flame resistant high strength fiber carrier has a first and a second surface shown respectively at 13 and 14 in FIG. 1.

In one embodiment, the carrier comprises from 40 to 70 weight percent of aramid fibers and from 30 to 60 weight percent of binder. In another embodiment, the carrier comprises from 40 to 55 weight percent of aramid fibers and from 45 to 60 weight percent of binder. A preferred binder is meta-aramid.

The thickness of the carrier used in this invention is dependent upon the end use or desired properties of the laminate but, to provide an overall high flexibility and the lowest possible weight, is typically from 1 to 7 mils (0.025 to 0.175 mm) or even from 3 to 7 mils (0.075 to 0.175 mm) thick. The carrier thickness may even be from 4 to 6 mils (0.100 to 0.150 mm). A carrier thickness below 1 mil would result in undesirable features such as a weaker and less dimensionally stable sheet, especially when saturated with water. A carrier having a thickness greater than 7 mils would add undesirable weight and stiffness.

In some embodiments, the carrier has a density of from 0.25 to 0.6 g/cc or from 0.25 to 0.45 g/cc or even from 0.30 to 0.40 g/cc. A carrier density of below 0.25 g/cc would result in undesirable features such as a weaker fluffy and excessively open structure. For papers with over 30 weight percent of polymeric binder, a carrier density of greater than 0.6 g/cc would require an excessive densification that would alter both surface properties (i.e. higher smoothness) and air permeability of the densified paper. Increased surface smoothness of the densified carrier would result in a lower release value potentially leading to inorganic refractory layer peeling off the densified carrier with a risk of breaks in the refractory layer. Lower air permeability would hinder the drying process of the coated paper.

In some embodiments, the basis weight of the carrier is from 0.59 to 2.06 ounces per square yard (20 to 70 grams per square meter).

The carrier has a surface smoothness on the surface that is in contact with the refractory layer of no less than 250 Sheffield units, Smoothness is concerned with the surface contour of paper. It is the flatness of the surface under testing conditions which considers roughness, levelness, and compressibility. This test is an indirect measure of paper smoothness or roughness. The Sheffield test method is a measurement of air flow between the test specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed in to the sample from top. Such a procedure is described in TAPPI T-538 om-08. In some embodiments, the carrier has a surface smoothness on at least one surface of from 300 to 400 Sheffield units.

The bond strength between the refractory layer and the surface of the paper is at least 0.25 lb/in, preferably at least 0.8 lb/in, If the bond strength is less than 0.25 lb/in, the inorganic refractory layer can peel off the carrier with a risk of breaks in the refractory layer. A bond strength of at least 0.8 lb/in ensures that the inorganic refractory layer does not separate from the carrier either during subsequent process steps or, once put in service, during the life span of the intended application. Bond strength is sometimes referred to as Release Value. In this instance, it is the Release Value between the surface of the paper and the intumescent coating applied to the paper.

The carrier has a wet tensile strength of at least 3 lb/in in a first direction and at least 2 lb/in in a second direction, the second direction being transverse to the first direction. In another embodiment, the paper has a wet tensile strength of at least 5 lb/in in a first direction and at least 2 lb/in in a second direction, the second direction being transverse to the first direction. In a preferred embodiment the first direction is the long direction within the plane of the paper, that is, the direction in which the roll of paper has been made. This is also known as the machine direction, The second direction is sometimes known as the cross direction. By wet tensile strength we mean the tensile strength of the paper after saturation with water. If the wet tensile strength is less than 3 lb/in in a first direction, there is a high risk of frequent sheet breaks during the coating process due to the weight being deposited on the paper and the tension applied to the paper.

The paper has a dry tensile strength of at least 7 lb/in in a first direction and at least 3 lb/in in a second direction, the second direction being transverse to the first direction. By dry tensile strength we mean the tensile strength of a paper that has been conditioned at ambient temperature and humidity, typically 48-52% Relative Humidity and 22-24 degrees C. TAPPI T-402 sp-08 is an example specification defining ambient conditions for paper, board and pulp products.

A dry tensile strength of at least 7 lb/in in a first direction is required to ensure proper handling of the coated web through the subsequent process c steps, in particular, to ensure tight roll formation during winding to prevent roll sagging and telescoping.

In some embodiments, the carrier has a dry tensile strength of at least 10 lb/in in the first direction and at least 5 lb/in in the second direction.

The carrier is hydrophilic. This feature aids the drying process. As the majority of the water from the refractory coating dispersion is absorbed by the carrier, this allows more efficient drying and forming of the inorganic refractory layer as well as preventing drying defects such as blisters in the refractory layer.

The carrier has an air permeability no greater than 600 Gurley Air Resistance (sec/100 cc, 20 oz. cyl). An air permeability of greater than 600 Gurley Air Resistance would adversely affect the drying process of the coated paper. In some embodiments, the paper has an air permeability of from 250 to 350 Gurley Air Resistance (sec/100 cc, 20 oz. cyl.).

The aramid fibers of the paper may be meta-aramid, para-aramid or a combination of the two.

The high temperature properties of the aramid fibers ensures thermal and mechanical stability of the carrier during processing steps when the carrier can be exposed to a temperature of 150 degrees C. for at least 10 minutes, that is to say, that the paper will not change dimensions when subjected to a temperature of 150 degrees C. for at least 10 minutes.

The aramid fibers of the paper can be in the form of floc, pulp, or a combination of thereof. As employed herein the term aramid means a polyamide wherein at least 85% of the amide (—CONH—) linkages are attached directly to two aromatic rings. Additives can be used with the aramid. In fact, it has been found that up to as much as 10 percent, by weight, of other polymeric material can be blended with the aramid or that copolymers can be used having as much as 10 percent of other diamine substituted for the diamine of the aramid or as much as 10 percent of other diacid chloride substituted for the diacid chloride of the aramid.

Floc is generally made by cutting continuous spun filaments into specific-length pieces. If the floc length is less than 2 millimeters, it is generally too short to provide a paper with adequate strength; if the floc length is more than 25 millimeters, it is very difficult to form uniform wet-laid webs. Floc having a diameter of less than 5 micrometers, and especially less than 3 micrometers, is difficult to produce with adequate cross sectional uniformity and reproducibility; if the floc diameter is more than 20 micrometers, it is very difficult to form uniform papers of light to medium basis weights.

The term “pulp”, as used herein, means particles of fibrous material having a stalk and fibrils extending generally therefrom, wherein the stalk is generally columnar and 10 to 50 micrometers in diameter and the fibrils are fine, hair-like members generally attached to the stalk measuring only a fraction of a micrometer or a few micrometers in diameter and 10 to 100 micrometers long. Aramid fiber floc is of a similar length to carbon fiber floc. Both meta and pars aramid fibers are suitable and are available from E.I. DuPont de Nemours, Richmond, Va. under the tradenames Kevlar® and Nomex® and from Teijin Twaron, Conyers, Ga. under the tradename Twaron®.

A preferred pulp material is p-aramid. However a blend of p-aramid with other synthetic or natural fibers such as liquid crystal polyester, polyareneazole, meta-aramid, and cellulose can be utilized. One illustrative process for making aramid pulp is disclosed in U.S. Pat. No. 5,084,136 to Haines et al.

Different thermoset and thermoplastic resins can be used as a polymeric binder in the paper of this invention. These resins can be supplied in the form of fibrids, flakes, powder, and floc. The term “fibrids” as used herein, means a very finely-divided polymer product of small, filmy, essentially two-dimensional, particles known having a length and width of 100 to 1000 micrometers and a thickness of 0.1 to 1 micrometer. Preferable types of binder resins are aramids, polyimides, phenolics, and epoxies. However, other types of the resins can also be used.

Fibrids are typically made by streaming a polymer solution into a coagulating bath of liquid that is immiscible with the solvent of the solution. The stream of polymer solution is subjected to strenuous shearing forces and turbulence as the polymer is coagulated. The fibrid material of this invention can be selected from meta or para-aramid or blends thereof. More preferably, the fibrid is a meta-aramid.

The paper can include small amounts of inorganic particles including mica, vermiculite, and the like; the addition of these performance enhancing additives being to impart properties such as improved fire resistance, thermal conductivity, dimensional stability, and the like to the paper and the final laminate.

In one preferred embodiment, the fiber and the polymer binder in the form of fibrids can be slurried together to form a mix that is converted to paper on a wire screen or belt. Reference is made to U.S. Pat. Nos. 4,698,267 and 4,729,921 to Tokarsky; U.S. Pat. No. 5,026, 456 to Hesler et al.; U.S. Pat. Nos. 5,223,094 and 5,314,742 to Kirayoglu et al for illustrative processes for forming papers from aramid fibers and aramid fibrids.

Once the paper is formed, it may be calendered to the desired void content/apparent density.

Inorganic Refractory Layer

The inorganic refractory layer is adjacent to at east one surface of the carrier. The refractory layer has a dry areal weight of from 15 to 50 gsm and a residual moisture content of no greater than 10 percent by weight. In some embodiments, the refractory layer has a dry areal weight of from 20 to 35 gsm and a residual moisture content of no greater than 3 percent by weight. The layer is shown as 12 in FIG. 1.

The refractory layer comprises platelets. Preferably at least 85% of the layer comprises platelets, more preferably at least 90% and most preferably at least 95%. In some embodiments, platelets comprise 100% of the layer. The refractory layer may comprise some residual dispersant arising from incomplete drying of the platelet dispersion during manufacture.

The refractory layer has a thickness of from 7.0 to 76 micrometers and more preferably from 7.0 to 50 micrometers. Preferably, the layer has a UL 94 flame classification of V-0. The function of the refractory layer, in which adjacent platelets overlap, is to provide a flame and hot gas impermeable barrier. The inorganic platelets may be clay, such as montmorillonite, vermiculite, mica, talc and combinations thereof. Preferably, the inorganic oxide platelets are stable (i.e., do not burn, melt or decompose) at about 600 degrees C., more preferably at about 800 degrees C. and most preferably at about 1000 degrees C. Vermiculite is a preferred platelet material. Vermiculite is a hydrated magnesium aluminosilicate micaceous mineral found in nature as a multilayer crystal. Vermiculite typically comprises by (dry) weight, on a theoretical oxide basis, about 38-46% SiO₂, about 16-24% MGO, about 11-16% Al₂O₃, about 8-13% Fe₂O₃ and the remainder generally oxides of K, Ca, Ti, Mn, Cr, Na, and Ba. “Exfoliated” vermiculite refers to vermiculite that has been treated, chemically or with heat, to expand and separate the layers of the crystal, yielding high aspect ratio vermiculite platelets. Suitable vermiculite materials are available from W. R. Grace of Cambridge, Mass., under the trade designations MicroLite 963 and MicroLite HTS-XE.

The thickness of an individual platelet typically ranges from about 5 Angstroms to about 5,000 Angstroms more preferably from about 10 Angstroms to about 4,200 Angstroms. The mean value of the maximum width of a platelet typically ranges from about 10,000 Angstroms to about 30,000 Angstroms. The aspect ratio of an individual platelet typically ranges from 100 to 20,000.

Preferably, the platelets have an average diameter of from 15 to 25 micrometers. In some other embodiments, the platelets have an average diameter of from 18 to 23 micrometers.

In a preferred embodiment, the refractory layer further comprises cations arising from contact, at a temperature of from 10 to 50 degrees C., with an aqueous cationic rich solution at a cation concentration of from 0.25 to 2N. The contact with the cationic solution occurs prior to assembling the refractory layer into a composite laminate. This cationic treatment provides enhanced stability to the refractory layer on exposure to fluids.

In some embodiments of this invention, the inorganic platelet layer is reinforced by a lightweight open weave fabric scrim either laid onto a single platelet layer or placed between two layers of platelets so as to provide additional mechanical strength to the layer. The scrim can be made from natural, organic or inorganic fibers with glass, cotton, nylon or polyester being typical examples. A glass fiber scrim is particularly preferred. The scrim may be a woven or knit structure and has a typical areal weight not exceeding 40 grams per square meter.

In some embodiments, the refractory layer is perforated to enhance bonding to an adhesive layer during subsequent processing. The extent of perforation is determined by experimentation. Preferably, in order to prevent compromising flame barrier properties, an individual perforation should not exceed 2 millimeters in maximum dimension. In a preferable embodiment, individual perforations should be spaced at least 10 millimeters apart. The shape of the perforations is not critical, Suitable perforations include circles, squares, rectangles, ovals and chevrons.

Method of Forming the Multilayered Sheet

A method of forming a layered sheet followed by subsequent treatment comprises the steps of

-   -   (i) depositing an aqueous slurry of inorganic refractory         platelets onto one surface of a carrier to form a layered sheet         wherein the refractory platelets     -   comprise from 7 to 13 weight percent of the slurry,     -   have a particle thickness of from 5 A to 5000 A,     -   have an average diameter of from 15 to 25 micrometers,         wherein the carrier has     -   (a) a wet tensile strength of at least 3 lb/in in a first         direction and at least 2 lb/in in a second direction, the second         direction being transverse to the first direction,     -   (b) a dry tensile strength of at least 7 lb/in in the first         direction and at least 3 lb/in in the second direction,     -   (c) a surface smoothness on at least one surface of no less than         250 Sheffield Units,     -   (d) a thickness of from 0.025 to 0.175 mm,     -   (e) a density of from 0.25 to 0.60 g/cc, and.     -   (f) has an air permeability no greater than 600 Gurley Air         Resistance (sec/100 cc, 20 oz. cyl),     -   (g) has a basis weight of from 20 to 70 gsm, and wherein the         carrier comprises from 40 to 70 weight percent of aramid fibers         and from 30 to 60 weight percent of polymeric binder, and

(ii) drying the layered sheet at a temperature of from 80 to 110 degrees C. until the residual moisture content in the refractory layer is no greater than 10 percent by weight and the bond strength between the refractory layer and the surface of the carrier is no less than 0.25 lb/in.

When the refractory platelets comprise from 11.5 to 13 weight percent of the slurry, it is preferable that the slurry is de-aerated (de-gassed) prior to deposition on the carrier. This will reduce defects due to trapped air.

Preferably, when the platelet content of the slurry is from 7.0 to 8.5 percent and the desired coat weight is 27 gsm or higher, then the coating is applied in multiple steps. For example, a 30 gsm total coat weight could be achieved by two applications of slurry each providing 15 gsm of refractory material or by three applications at 10 gsm.

In some embodiments, the layered sheet is dried at a temperature of from 80 to 110 degrees C. until the residual moisture content in the refractory layer is no greater than 3 percent by weight. In some other embodiments, the method comprises an optional step of increasing the drying temperature in step (ii) to from 150 to 180 degrees C. after the residual moisture content in the refractory layer is less than 10 percent.

In some embodiments, the refractory platelets comprise from 10 to 11 weight percent of the slurry.

Preferably the layered sheet, when wet, has a shrinkage of no greater than 2 percent.

Prior to coating the carrier with refractory material, the carrier may optionally be treated to promote better wetting. An example of such a treatment is plasma or corona treatment.

Use of the Refractory Layer

The layered sheet may be used as a component in a flame barrier layer for a thermal insulation and acoustic blanket. An example of such a blanket is described in United States patent application publication 2011/0094826.

Test Methods

The wet tensile strength of the carrier was measured according to TAPPI T456 om-10 Tensile Breaking Strength of Water-saturated Paper and Paperboard (“Wet Tensile Strength”).

The dry tensile strength of the carrier was measured according to TAPPI T494 om-06 Tensile Properties of Paper and Paperboard (Using Constant Rate of Elongation Apparatus).

The surface smoothness of the carrier was measured according to TAPPI T538 om-08 Roughness of Paper and Paperboard (Sheffield Method),

The thickness of the carrier was measured by TAPPI T411 om-10 Thickness (Caliper) of Paper, Paperboard, and Combined Board.

The density of the carrier is a calculated value based on the measured values of carrier thickness and basis weight.

The air permeability of the carrier was measured according to TAPPI T460 am-11 Air Resistance of Paper (Gurley Method, sec/100 cc, 20 oz. cyl.).

The dimensional stability of the carrier was rated based on its ability to hold flat (i.e. no moisture related wrinkles or creases) for at least 2 minutes when exposed to one-sided wetting.

The dry areal weight of the refractory layer was measured according to ISO 536 (1995) Determination of Grammage and TAPPI T 410 Grammage of Paper and Paperboard (Weight per Unit Area).

The moisture content of the refractory layer was measured according to ISO 287 (1985) Determination of Moisture Content—Oven Drying Method.

Two-layer composite sheets were subjected to a flame test that replicated the temperature and air mass flux test conditions of test method FAA FAR 25.856(b), App. F, Part VII. The somewhat lower heat flux was compensated with a higher air mass flux to replicate a required thermo-mechanical stress level to be exerted on the flame barrier composites during the burn-through test.

EXAMPLES

In the following examples, all parts and percentages are by weight and all degrees in centigrade unless otherwise indicated. Examples prepared according to the current invention are indicated by numerical values. Control or Comparative Examples are indicated by letter

The vermiculite used was a high solids version of an aqueous dispersion of Microlite® 963 having an as supplied solids content of 7.5 percent. The dispersion was obtained from W.R. Grace and Co, Cambridge, Mass.

Example 1

Vermiculite dispersion concentrated to a solids content of 10.6% weight percent was coated on 5 mil thick meta-aramid paper using a slot die coating system to form a refractory layer on the paper. The paper was 5 mil Nomex® paper from DuPont. The paper comprised from 45 to 50 weight percent of meta-aramid fiber floc and from 50 to 55 weight percent of polymeric binder in the form of fibrids.

The paper had a basis weight of 1.23 oz/sq. yd., an average thickness of 4.9 mil, a density of 0.34 g/cc, a Gurley Air Resistance of 316 sect 100 cc, 20 oz. cyl., a smoothness of 325 Sheffield units, a dry tensile strength of 10.7 lb./in. in the machine direction and 5.5 lb./in. in the cross direction. The wet tensile strength was 5.1 lb/in, in the machine direction and 2.95 lb./in, in the cross direction. The coated paper was dried for 15 minutes in an air flotation oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. Differential drying temperatures were applied to the top (vermiculite side) and the bottom (paper side). The drying profile on the top side was 5 minutes at 49 degrees, 5 minutes at 60 degrees and 5 minutes at 71 degrees. The drying on the bottom side was maintained for 15 minutes at 99 degrees. The refractory layer had a dry coat weight of 37 gsm. The two layer composite of paper and refractory layer was wound up on a roll.

The inorganic refractory layer on the aramid paper formed an effective lightweight and flexible 2-layer composite. There were no practical ways to remove any substantial sections of the refractory layer from the paper base without the aid of a reinforcing substrate bonded to the exposed side of the refractory film layer, and even this could only be achieved with difficulty. The inorganic refractory material remained attached to the surface of the aramid paper even after substantial flexing.

The 2-layer composite was subjected to a flame test that replicated the temperature and air mass flux test conditions of test method FAA FAR 25.856(b), App. F, Part VH. When exposed to a flame on the inorganic refractory layer side, the sample showed a good resistance to flame propagation, with the inorganic refractory layer acting as an effective flame barrier.

Comparative Example A

Vermiculite dispersion concentrated to a solids content of 10.6 weight percent was coated on 2-mil thick metalized polyester film using a slot die coating system to form a refractory layer on the film. The film was metalized on one side. The coating was applied to the metalized side of the film. The film was obtained under the tradename Mylar from E.I. DuPont de Nemours and Co., Wilmington, Del. The coated film was dried in an oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. The total drying time exceeded 75 minutes comprising a staged drying of 15 minutes at 60 degrees, 15 minutes at 71 degrees, 15 minutes at 82 degrees, 15 minutes at 93 degrees, and over 15 minutes at 99 degrees. The refractory layer had a dry coat weight of 35 gsm. The paper and refractory layers were wound up on as separate rolls.

From inspecting a sample of the two layer composite, it was observed that the dried refractory layer spontaneously peeled away from the metalized side of the film. Although the peel characteristics were good, with 0.5 lbs/in tensile strength, the unsupported inorganic refractory film-like material was extremely difficult to handle and extra care had to be taken to further process the material.

Comparative Example B

Vermiculite dispersion concentrated to a solids content of 13 weight percent was coated on a 6 micron thick polyetheretherketone (PEKK) film using a slot die coating system to form a refractory layer on the film. The film was grade DS-E obtained from Cytec Industries, Woodland Park, N.J. The coated film was dried in an oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. The drying time exceeded 45 minutes comprising a staged drying of 9 minutes at 71 degrees, 6 minutes at 82 degrees, 6 minutes at 93 degrees, and 25 minutes at 96 degrees. The refractory layer had a dry coat weight of 33 gsm. The two layer composite of film and refractory layer was wound up on a roll.

The coating process proved to be very difficult due to tendency for the film to wrinkle and crease. Further, the film had to be surface treated by a process such as corona treatment to promote wetting and give a uniform coating, Although relatively continuous refractory layer coating was obtained the refractory layer was highly non-uniform and affected by streaks and light spots related to excessive air bubbles trapped in the high viscosity solution.

Comparative Example C

Vermiculite dispersion concentrated to a solids content of 7.5 weight percent was coated on 0.5 mil thick polyimide film using a knife over roll coating system to form a refractory layer on the film. The film was obtained under the tradename Kapton from E.I. DuPont de Nemours and Co., Wilmington, Del. The coated film was dried in an oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. The drying time exceeded 75 minutes comprising a staged drying of 20 minutes at 71 degrees, 20 minutes at 82 degrees, 20 minutes at 93 degrees, and over 25 minutes at 96 degrees. The refractory layer had a target dry coat weight of 33 gsm. The two layer composite of film and refractory layer was wound up on a roll.

The coating process proved to be very difficult due to an extremely low viscosity of the coating solution combined with tendency for the film to wrinkle and crease. Further, the film had to be surface treated by a process such as corona treatment to promote wetting and give a uniform coating, A uniform and continuous refractory layer coating was not obtained.

Comparative Example D

Vermiculite dispersion concentrated to a solids content of 10.8 weight percent was coated on 2 mil thick polyimide (Kapton®) film using a slot die coating system to form a refractory layer on the film. The coated film was dried in an oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. The drying time exceeded 75 minutes comprising a staged drying of 9 minutes at 71 degrees, 6 minutes at 82 degrees, 6 minutes at 93 degrees, and 60 minutes at 96 degrees. The refractory layer had a dry coat weight of 33 gsm. The two layer composite of film and refractory layer was wound up on a roll.

Once dried to below 5% moisture content, a very uniform and continuous refractory layer resulted. The layer remained on the surface of the film with enough adhesion to allow for smooth roll winding and post-processing. Refractory layer was easily peeled off the polymeric film base with a help of a reinforcing substrate that was bonded to the exposed side of the refractory film. It was also possible to peel substantial sections of the refractory layer off the polymeric film base without the aid of a reinforcing substrate; however extreme care has to be taken to prevent premature breaks of the film-like refractory layer.

When exposed to a flame on the inorganic refractory layer side, the sample showed a good resistance to flame propagation, with the inorganic refractory layer acting as an effective flame barrier.

However, the drying time for a coating process in excess of 75 minutes was too long to be of practical value. Further, the inorganic refractory material showed signs of localized delamination/detachment from the polymeric film base when flexed.

Comparative Example E

This was as Example A except that the film layer did not have a metalized surface. The findings were the same as for Comparative Example D, with the exception for flame propagation properties. When exposed to a flame on the inorganic refractory layer side, an inorganic refractory layer acted as an effective flame barrier, however the overall 2-layer composite propagated fire on the polymeric film side.

Comparative Example F

Vermiculite dispersion was coated on 5.6 mil thick reinforced polyethylene sheet using a doctor blade. The polyethylene sheet was Tyvek® grade 1056D from DuPont. The coated sheet was dried in an oven at 90 degrees C. until the refractory layer had moisture content below 5%. The drying time was 30 minutes. The dry basis weight of of the refractory layer was 37 gsm.

The dried refractory layer could not be removed from the sheet even with the help of a reinforcing substrate bonded to the exposed side of the refractory layer. Cohesive bond failure within the refractory layer was observed. The polyethylene sheet was unsuitable for use.

Comparative Example G

Vermiculite dispersion concentrated to a solids content of 10.8% weight percent was coated on 11 mil thick hydrophilic gray RagKraft paper using a slot die coating system to form a refractory layer on the paper. The paper comprised a blend of 50 weight percent of cellulose fibers and 50 weight percent of cotton fibers and was obtained from Crocker Technical Papers.

The paper had a basis weight of 8.1 oz/sq. yd., an average thickness of 11.0 mil, a density of 1.0 cc, a Gurley Air Resistance of 714 sec/100 cc, 20 oz. cyl., a smoothness of 103 Sheffield units, a dry tensile strength of 122.0 lb/in. in the machine direction and 40.0 lb./in. in the cross direction. The wet tensile strength was 6.4 lb./in. in the machine direction and 2.5 in the cross direction.

The coated paper was dried for 15 minutes in an air flotation oven at a temperature not exceeding 110 degrees C. until the inorganic refractory layer had moisture content below 5%. Differential drying temperatures were applied to the top (vermiculite side) and the bottom (paper side). The drying profile on the top side was 5 minutes at 49 degrees, 5 minutes at 60 degrees and 5 minutes at 71 degrees. The drying on the bottom side was maintained for 15 minutes at 99 degrees. The refractory layer had a dry coat weight of 33 gsm. The two layer composite of film and refractory layer was wound up on a roll.

Once dried to below 5% moisture content, a very uniform and continuous refractory layer resulted. The layer remained on the surface of the paper with enough adhesion to allow for smooth roll winding and post-processing. The refractory layer was easily peeled off the paper base with a help of reinforcing substrate that was bonded to the exposed side of the refractory film. With extreme care it was also possible to peel short sections of the refractory layer from the paper base without the aid of a reinforcing substrate.

When exposed to a flame on the inorganic refractory layer side, the refractory layer acted as an effective flame barrier, however the overall 2-layer composite propagated fire on the paper side, 

What is claimed is:
 1. A layered sheet comprising a carrier having a first and second surface and an inorganic refractory layer adjacent to at least one surface of the carrier wherein the refractory layer has a dry areal weight of from 15 to 50 gsm, the bond strength between the refractory layer and the surface of the carrier is no less than 0.25 lb/in, and wherein the carrier (i) comprises from 40 to 70 weight percent of aramid fibers and from 30 to 60 weight percent of polymeric binder, (ii) is hydrophilic (iii) has a wet tensile strength of at least 3 lb/in in a first direction and at least 2 lb/in in a second direction, the second direction being transverse to the first direction, (iv) has a dry tensile strength of at least 7 lb/in in a first direction and at least 3 lb/in in a second direction, the second direction being transverse to the first direction, (v) has an air permeability no greater than 600 Gurley Air Resistance (sec/100 cc, 20 oz. cyl.). (vi) has a smoothness on at least one surface of no less than 250 Sheffield units. (vii) has a thickness of from 0.025 to 0.175 mm, (viii) has a density of from 0.25 to 0.60 g/cc, and (ix) has a basis weight of from 20 to 70 gsm.
 2. The layered sheet of claim 1 wherein the refractory layer has a residual moisture content of no greater than 10 percent by weight.
 3. The layered sheet of claim 1 wherein the inorganic refractory layer comprises vermiculite.
 4. The layered sheet of claim 1 wherein the aramid fiber comprising the paper is meta-aramid, para-aramid or a combination thereof.
 5. The layered sheet of claim 1 wherein the layered sheet, when wet, has shrinkage no greater than 2 percent.
 6. The layered sheet of claim 1 wherein the refractory layer has a dry areal weight of from 20 to 35 gsm.
 7. The layered sheet of claim 1 wherein the paper has a wet tensile strength of at least 5 lb/in in a first direction and at least 2 lb/in in a second direction, the second direction being transverse to the first direction.
 8. The layered sheet of claim 1 wherein the paper has a dry tensile strength of at least 10 lb/in in a first direction and at least 5 lb/in in a second direction, the second direction being transverse to the first direction.
 9. The layered sheet of claim 1 wherein the paper has an air permeability of from 250 to 350 Gurley Air Resistance (sec/100 cc, 20 oz. cyl.).
 10. The layered sheet of claim 1 wherein the carrier has a thickness of from 0.075 to 0.175 mm (3-7 mil).
 11. The layered sheet of claim 1 wherein the carrier has a thickness of from 0.100 to 0.150 mm (4 to 6 mil).
 12. The layered sheet of claim 1 wherein the carrier has density of from 0.25 to 0.45 g/cc.
 13. The layered sheet of claim 1 wherein the carrier has a density of from 0.30 to 0.40 g/cc.
 14. The layered sheet of claim 1 wherein the carrier is thermally stable at 150 degrees C. for at least 10 minutes.
 15. The layered sheet of claim 1 wherein the bond strength between the refractory layer and the surface of the carrier is at least 0.8 lb/in. 